The present invention relates to the field of overvoltage protection devices, and more particularly, to a gas filled spark gap device which switches high current from a charged capacitor into an exploding bridge wire detonator at a precise breakdown voltage.
In the art of high voltage, high current circuit applications, spark devices are known to protect circuit components, such as an energy storage capacitor, from voltage overloads, and to switch current from charged capacitors into output loads at various breakdown voltages. The overvoltage gap devices are required to remain inactive until breakdown voltage conditions are reached, to switch the high voltage, high current electrical surges rapidly, and to return to normal quickly after the breakdown condition has passed in readiness for subsequent operations.
Certain problems arise in the operation of spark gap devices described in the prior art. For example, in U.S. Pat. No. 2,990,492 of Wellinger et al, a gas-filled spark gap device is disclosed that is generally operable to provide overload protection, but has an undesirable short life span. It has been found in practice, the metal electrodes may erode during extended operation and cause a metallic deposit to form on ceramic insulator surfaces rendering them conductive thereby significantly reducing the overall lifetime of the device. Furthermore, to assist in ionizing the spark gap at a stable breakdown voltage level, a radioactive gas, krypton-85, is employed. The radioactive gas may leak and cause an undesirable release of radioactivity into the environment.
In U.S. Pat. no. 3,317,777 of Algar et al, a high pressure mixture of xenon and nitrogen is used for a gas-filled spark gap device. During high energy electrical discharge, nitrogen is a reactive gas and causes undesirable deterioration of the spark gap electrodes. Copper electrodes are employed. The melting point of copper is a relatively low 1,083.degree. C.
The structure of a voltage switching device for a high voltage, high current application, as described, is subject to and thus must withstand electrode temperatures in excess of 2,000.degree. C. in many instances. Prior art electrodes, such as shown in Algar et al, are thus incapable of withstanding this temperature without serious erosion, and this fact greatly reduces the efficiency of the device.
In the spark gap device disclosed by Kawiecki in U.S. Pat. No. 3,588,576, the electrodes defining the spark gap are made from a nickel and cobalt alloy. The nickel-cobalt alloy electrodes melt in the range of approximately 1,450-1,500.degree. C. and thus are also generally ineffective. Inside the spark gap device, conductive strips are connected to the electrodes and extend along the inner wall of the ceramic spacer to a location opposite the electrode gap. The strips serve to stabilize operating characteristics and speed response time. Being inside the device, however, the stabilizing strips are subjected to deteriorating conditions during device discharge.
Spark gap devices are often encapsulated in organic materials for protection. However, during aging on the shelf, organic materials may emit hydrogen gas which is able to permeate through nickel, copper, nickel-cobalt alloys into the sealed chamber and change the fill gas purity.